1. Field of the Invention
The present invention generally relates to an integrated multiport antenna for increasing information rates of wireless communication networks.
2. Description of the Related Art
One of the more critical pieces of equipment in a communication network and, in particular, in a wireless communication network is the antenna. Antennas are used to convey information (i.e., transmit and receive information) in the form of electromagnetic waves over communication links of a network.
The owners and/or operators of communication networks, i.e., the service providers, are constantly searching for methods and equipment that can meet the changing needs of their subscribers. Subscribers of communication networks, including wireless communication networks, require higher information throughput in order to exploit the expanding range of services being provided by current communication networks. For example, wireless communication subscribers are now able to have simultaneous access to data networks such as the Internet and to telephony networks such as the Public Switched Telephone Network (PSTN). Also, service providers are constantly investigating new techniques that would allow them to increase their information throughput. Information throughput is the amount of informationxe2x80x94usually measured in bits per secondxe2x80x94successfully conveyed (transmitted and received) over a communication channel. Information throughput can be increased in a number of well known manners. One way is by increasing the power of the transmitted signals. A second way is by expanding the frequency range (i.e., bandwidth) over which the communication is established. However, both power and bandwidth are limited by certain factors such as governmental and standards organization that regulate such factors. In addition, for portable devices, power is limited by battery life.
An approach which circumvents the power and bandwidth limitations is to increase the number of antennas used to transmit and receive communication signals. Typically, the antennas are arranged as an array of antennas. Three of the more general ways of using antenna arrays are (a) phased array applications (b) spatial diversity techniques and (c) Multiple Input Multiple Output (MIMO) techniques. A phased array comprises an antenna array coupled to a device, which controls the relative phase of the signal in each antenna in order to form a focused beam in a particular direction in space. Spatial diversity is the selection of a particular antenna or a group antennas of the array to transmit or receive signals in order to improve information throughput. In a spatially diverse structure the antenna array is typically coupled to a receive diversity device that utilizes one of many combining techniques, such as Maximum Ratio Combining, switching, or many others well known to those skilled in the art. Unlike phased arrays and spatial diversity techniques wherein one or a group of antennas are used to transmit and/or receive a single signal, a technique called Multiple Input Multiple Output (MIMO) is used whereby the antenna array coupled to a signal processing device is used to transmit and/or receive multiple distinct signals. One example of a MIMO system is the BLAST (Bell Labs LAyered Space Time) system conceived by Lucent Technologies headquartered in Murray Hill, N.J.
In many cases, as the number of antennas in a transmit and/or receive array (e.g., BLAST system) is increased, the information throughput of the system also increases; G. J. Foschini and M. Gans, Wireless Commun. 6, 311 (1998). Typically the amount of space available for the antenna array is limited. In particular, the space limitation is very critical for portable wireless devices (e.g., cell phones, Personal Digital Assistants (PDA)). Thus, increasing the number of antennas in an array of limited space decreases the spacing between individual antennas in the array. The reduced spacing between antennas typically causes signal correlation to occur between signals received from different antennas. Signal correlation reduces the gain in information throughput obtained by the use of MIMO techniques; A. L. Moustakas et al., Science 287, 287 (2000).
In particular, received signal correlation is a phenomenon whereby the variations in the parameters (i.e., amplitude and phase) of a first signal of a first antenna track the variations in the parameters of a second signal of a second antenna in the vicinity of the first antenna; Microwave Mobile Communications, W. J. Jakes (ed.), chapter 1, IEEE Press, New York (1974). Correlation is quantitatively defined in terms of at least two signals. When any two signals s1(t) and s2(t) are being transmitted or received, the degree of correlation between these two signals is given by the absolute value of the following expression:             ∫              t        1                    t        2              ⁢                  s        1            ⁢              (        t        )            ⁢                        s          2                ⁡                  (          t          )                    *              xe2x80x83            ⁢              ⅆ        t                                ∫                  t          1                          t          2                    ⁢                                    "LeftBracketingBar"                                          s                1                            ⁢                              (                t                )                                      "RightBracketingBar"                    2                ⁢                  xe2x80x83                ⁢                  ⅆ          t                ⁢                              ∫                          t              1                                      t              2                                ⁢                                                    "LeftBracketingBar"                                                      s                    2                                    ⁢                                      (                    t                    )                                                  "RightBracketingBar"                            2                        ⁢                          xe2x80x83                        ⁢                          ⅆ              t                                          
where s2*(t) corresponds to the complex conjugate of s2(t) and t1 and t2 are times selected in accordance to rules well known to those skilled in the pertinent art. When two signals have low correlations or are uncorrelated, the above integral becomes relatively-small.
The correlation between received signals can be determined by the correlation of the radiation patterns of the antennas receiving the signals. As it is known to those skilled in the art, the radiation pattern of a particular antenna or cluster of antennas fed through a port, is the relative amplitude, direction and phase of the electromagnetic field in the far field region radiated at each direction. The radiation patterns are reciprocal in that they show the relative amplitude, phase and direction of a field transmitted from an antenna as well as the sensitivity of that antenna to incoming radiation from the same direction. The radiation pattern can be measured experimentally in an anechoic chamber, or calculated numerically with the use of a programmed computer.
The correlation function of two radiation patterns is a useful measure of the degree of their overlap. It is defined as the magnitude of       ∫                  ⅆ        k            ⁢                                                  E              ⇀                        1                    ⁢                      (            k            )                          ·                              (                                                            E                  ⇀                                2                            ⁢                              (                k                )                                      )                    *                                ∫                        ⅆ          k                ⁢                              "LeftBracketingBar"                                                            E                  ⇀                                1                            ⁢                              (                k                )                                      "RightBracketingBar"                    2                ⁢                  ∫                                    ⅆ              k                        ⁢                                          "LeftBracketingBar"                                                                            E                      ⇀                                        2                                    ⁢                                      (                    k                    )                                                  "RightBracketingBar"                            2                                          
where E1(k) and E2(k) are the far field vector electric fields at direction k of the radiated field at a given frequency due to ports 1 and 2 respectively and E2(k)* is the complex conjugate of the far field vector electric field at direction k due to port 2. The correlation between radiation patterns can be calculated based on the experimentally determined or numerically calculated individual-radiation patterns.
When two antennas are placed sufficiently far from each other, the correlation of their radiation patterns at the same frequency will be very small. A result of this effect is that the received signal from two antennas spaced sufficiently apart will be independent. The radiation pattern of a port of an antenna generally depends on many factors. A port is a part of the antenna at which a signal is applied to produce electromagnetic radiation or a point on the antenna from which a signal is obtained as the result of electromagnetic radiation impinging on the antenna. The factors affecting the radiation pattern of a port of an antenna include the placement of the port, the materials from which the port and antenna are constructed, structure and shape of the antenna, the relative position of the antenna in an antenna array, the relative position of the antenna within a communications device, as well as the position of other objects proximately spaced to the antenna. The reason for this dependence is the electromagnetic coupling of the antenna to nearby objects. In general, electromagnetic coupling of an antenna to other objects or other antennas can modify the radiation pattern of one or more of the ports of the antenna.
The radiation pattern at a particular frequency of a particular port of a particular antenna in a particular antenna array has several well known characteristics. One such characteristic is a node or a null. A node or a null is a direction in space where the transmitted (or received) radiation power is zero or relatively small, i.e., more than 20 dB below the average radiated power. Another property is a lobe which is a direction in space where the radiated power has a xe2x80x98local maximumxe2x80x99. A direction in space where the radiated power is at its highest measured value (commonly referred to as xe2x80x98absolute maximumxe2x80x99) is called the main lobe of the port. A lobe generally has a width, corresponding to the directions around it that have appreciable radiated power. The width of the lobe is defined as the set of directions in the immediate neighborhood of the lobe maximum which has a radiated power of more than half the radiated power of lobe maximum. Also, two lobes from two different radiation patterns at the same frequency are considered as not overlapping if their respective widths do not overlap.
It is useful to describe the radiation pattern in terms of the radiation pattern of an ideal dipole antenna since many antennas have patterns that are similar to those of dipole antennas. A dipole radiation pattern is defined to have a null in two opposite collinear directions and a peak radiated power in the plane perpendicular to the collinear direction, with the power in that plane fluctuating by no more than 5 dB. Such a radiation pattern is said to be polarized along the axis of the nulls. When two ports of an antenna have dipole radiation patterns that have null axes with relative angles higher than 20 degrees, the antenna is dually polarized a given frequency when only these 2 ports are operating at that frequency. If the dually polarized antenna has axes with relative angles between 70 and 110 degrees, it is said to be cross-polarized. Similarly, if m ports of an antenna, with m greater or equal to 3, have dipole radiation patterns, such that any two axes have a relative angle greater than 20 degrees, then the antenna is m-fold polarized at a given frequency when all m ports are operating at that frequency.
Typically the antennas are at least on the order of a wavelengh-apart. A wavelength of a signal is the ratio of the speed of light in vacuum to the frequency of the signal. For example a signal having a frequency of f has a wavelength xcex equal to c/f where c is a well known physical constant representing the speed of light in vacuum which is approximately   3  xc3x97      10    8    ⁢      xe2x80x83    ⁢            m              sec        .              .  
It is well known that the correlation between received signals of approximately placed antennas increases as the antennas are placed closer to each other; Microwave Mobile Communications, W. J. Jakes (ed.), chapter 1, IEEE Press, New York (1974). In the case of two antennas receiving signals, as the signals being received by the antennas become more correlated, the use of the second antenna becomes essentially redundant; this is because both antennas are receiving, in essence, the same information since the information carried by a signal is typically encoded with the variations in one or more parameters, (i.e., amplitude, phase) of a signal. Conversely, when the signals being received from the different antennas are uncorrelated, the signals are independent of each other and therefore the antenna system can receive information up to twice the information rate of one antenna alone; G. J. Foschini and M. Gans, Wireless Commun. 6, 311 (1998).
It is also well known that in order to avoid the type of correlation that render one or more nearby antennas redundant, the distance between the antennas should be at least xcex/2 where xcex is equal c/f to is the wavelength corresponding to the largest frequency f within a band of frequencies being used for communication by the antennas; Microwave Mobile Communications, W. J. Jakes (ed.), chapter 1, IEEE Press, New York (1974). The need to,have a group of antennas or an array of antennas situated in a relatively small space, while maintaining a relatively low degree of antenna correlations, is a critical problem for many communication devices, particularly wireless mobile communication devices.
One approach that has been proposed for packing many antennas into a small space is to construct an array of individual antennas; Vaughan et al., U.S. Pat. No. 5,771,022; xe2x80x9cClosely Spaced Monopoles for Mobile Communicationsxe2x80x9d, Rodney G. Vaughan and Neil L. Scott, Radio Science vol. 28, Number 6, Pp 1259-1266 (1993). In this antenna array approach, several individual antennas with various desirable engineering properties (e.g., high gain, lightweight, small, manufacturable), are assembled into an antenna array. It is found that under certain circumstances individual antennas can be spaced a small fraction of xcex (less than 0.2 xcex, for example) and due to the electromagnetic coupling between the antennas, the correlation between signals received at the two antennas can remain smaller than 0.7. The antenna array approach, however, has several disadvantages. One disadvantage is that the available space on a mobile device or any other device may not be shaped to allow an array of individual antenna elements to be positioned therein. In addition, construction of antenna arrays from individual antenna elements may result in other undesirable features of the antenna array including poor gain, low durability, or relatively high manufacturing costs. More generally, the problem associated with f constructing antenna arrays is that the variety of antenna arrays that one can fashion out of composites of individual antennas is limited which therefore limits the flexibility in designing such antenna arrays for communication devices.
What is therefore needed is an antenna used for various applications including wireless communication applications where such an antenna device: (a) occupies a relatively small volume of space, (b) maintains low correlations between the different signals transmitted and/or received by such a device and (c) enables great flexibility in designxe2x88x92allowing a large variety of shapes and structures so that the antenna design satisfies various engineering constraints (e.g., size, desired shape, manufacturing costs).
The present invention provides an integrated multi-port antenna that occupies a relatively small volume and is capable of transmitting and/or receiving uncorrelated signals for achieving high information throughput. Further, the integrated nature of the structure allows great flexibility in design for satisfying various engineering requirements such as size, shape, ease of manufacture and durability.
In particular, the present invention provides method and apparatus for an integrated K-port antenna where K is an integer equal to 2 or greater where the antenna can be enclosed by Kxe2x88x921 overlapping spheres each having a diameter xcex/2 where xcex is equal to c/f and f is the lowest frequency component of a range of frequencies within which each of the ports operates. The correlation between the radiation patterns produced by any two ports is less than 0.7
In a preferred embodiment, non metallic materials with relatively high dielectric constants are used to allow the construction of K-port antennas of smaller size with improved radiation characteristics.